EVEN BEFORE HE'D FALLEN terminally ill in 2010, Christopher Hitchens — a writer legendary for rapidly executing his deadlines on the fly — had begun writing far more deliberately with posterity in mind. In 2007, he’d published his systematic defense of atheism, God Is Not Great — a polemic summing-up of one of his longest-running cultural arguments that Hitchens regarded as his best work; three years later, he published a memoir, Hitch-22, to wide critical acclaim and a spot on the New York Times bestseller list.

It was at the outset of his publicity tour Hitch-22 that Hitchens learned the awful news that he had become gravely ill. He’d been slated to appear on “The Daily Show with Jon Stewart,” and then at the 92^nd Street Y, where he was to be interviewed about the book by his old friend Salman Rushdie. After a brutal morning tour in the emergency room, he recounts in Mortality that he dutifully hit his mark at both events, “though I did vomit two times, with an extraordinary combination of accuracy, neatness, violence, and profusion, just before each show. This is what citizens of the sick country do while they are still hopelessly clinging to their old domicile.” Within a week, he learned that he had esophageal cancer — an exceptionally lethal form of the disease that also claimed his father’s life — and that the prognosis for recovery was exceedingly bad. Posterity, in other words, was not going to wait much longer.

Mortality is not the sustained Socratic mediation on the human condition that the title might suggest. It is, rather, made up largely of just this sort of carefully reported, drily ironic dispatches from the sick country (or “Tumorland,” as Hitchens comes to call it) — meticulously recording both the physical symptoms of rapidly encroaching decay, and the feeble human effort to assimilate them into whatever semblance of a recognizably normal life may still remain. At its heart, this slender volume is a prolonged and painful study in cognitive dissonance, as the robust, high-living and (yes) terminally witty Hitchens records the galloping dissolution of his health and consciousness — the two things that humans almost have to take for granted in order to function in any reliable fashion. If, as Montaigne famously said (by way of Cicero) “to study philosophy is to learn to die,” Mortality is a crash course in lived philosophy, without benefit of abstraction or metaphysical speculation.

Which is not to say, of course, that Hitchens refrains from going another round with his antagonists on the question of religious belief. At one point, he alights upon a Web contribution from a devout Christian gloating over what he took to be a cunning divine plan to smite Hitchens down, one of the faith’s most vocal critics — nothing less, the gentle correspondent divined, than “God’s revenge for him using his voice to blaspheme him.” Hitchens offers a typically patient, yet scornful reply:

Devout persons have died young and in pain. Bertrand Russell and Voltaire, by contrast, remained spry until the end, as many psychopathic criminals and tyrants have also done. These visitations, then, seem awfully random. My so far uncancerous throat, let me rush to assure my Christian correspondent above, is not at all the only organ with which I have blasphe...